The present invention relates to the use of silver alloys as cadmium-free brazing solders, in particular as vacuum brazing solders and as general purpose brazing solders for use with fluxes in air.
Brazing solders are considered to be metal alloys suitable for brazing with working temperatures in excess of 450.degree. C. They often consist of silver with added copper, zinc and cadmium. Currently there are practically no usable general purpose brazing solders for use with fluxes, or under a vacuum or protective gas, which have a working temperature below 600.degree. C. However, the lower the working temperature, the less the mechanical properties of the workpieces to be joined are impaired during the brazing operation by heating to this temperature and energy requirements are simultaneously reduced. It is therefore important to have available brazing solders which have the lowest possible working temperature.
Brazing solders with working temperatures between 600.degree. and 700.degree. C. mainly consist of alloys of the metals silver, copper, tin, zinc, and cadmium. It is, however, known that cadmium and its highly volatile oxides may have toxic effects if they are absorbed by the human body. Such absorption cannot always be completely eliminated when brazing is performed incorrectly with solders containing cadmium, so that there may be a risk of poisoning. This gives rise to the requirement to greatly reduce the cadmium content of brazing solders or to keep brazing solder alloys free of cadmium.
In most previously used brazing solder alloys with a working temperature between 600.degree. and 700.degree. C., a moderate or high cadmium content is necessary to achieve these low temperatures. Previously known cadmium-free brazing solders have working temperatures which are 80.degree. to 120.degree. C. higher than for solders containing cadmium, which is not tolerable for temperature sensitive materials. While brazing solder alloys of silver-copper-tin with high proportions of tin do have low working temperatures, they are, however, very brittle and cannot be made into shaped articles.
A cadmium-free brazing solder based on silver is known from DE-AS 24 17 060, which contains 40 to 50 wt. % silver, 15 to 38 wt. % copper, 22 to 32 wt. % zinc, 1 to 6 wt. % tin, and 0.5 to 3 wt. % indium. The working temperatures of these brazing solders are between 710.degree. and 630.degree. C., which is, however, still too high for various applications.
DE-OS 33 15 498 describes stratified contact pieces for low current contacts in which the solder layer consists of a silver-copper-gallium alloy. The alloy contains 60 to 75 wt. % silver, 18 to 35 wt. % copper, and 5 to 8 wt. % gallium, wherein the latter component may also be replaced with 4 to 7 wt. % gallium plus 1 to 4 wt. % indium or with 1 to 4 wt. % gallium and 3 to 7 wt. % tin. Melting points for these solders are not stated; they are, however, above 650.degree. C.
Solders with 50 to 65 wt. % silver, 5 to 41 wt. % copper, 3 to 12 wt. % gallium, and 6 to 18 wt. % indium are known from Soviet patent SU 450 673 (Derwent Abstract 75-65066W7/39). Their melting point is from 640.degree. to 680.degree. C.
DE-OS 27 45 409 describes brazing solder alloys with 50 to 70 wt. % silver, 15 to 30 wt. % copper, 8 to 20 wt. % zinc, and 0.1 to 8 wt. % gallium and/or indium. Their melting point is between 650.degree. and 680.degree. C.
The melting points of these alloys may be still further reduced by raising the gallium content. However, silver-copper alloys with more than 8 wt. % gallium are difficult to work and may no longer be made into semi-finished articles.